Ultrafast Molecular Dynamics Probed by Coherent Electrons ...online.itp.ucsb.edu › online ›...

Ultrafast Ultrafast Molecular Dynamics Probed byMolecular Dynamics Probed byCoherent Electrons and X-RaysCoherent Electrons and X-Rays

Nick Wagner, Andrea Wüest, Robynne Hooper, Xibin Zhou, Zach Walters, StefanoTonzani

Ivan Christov, Chris Greene, Margaret Murnane and Henry Kapteyn

Outline

• Probe internal dynamics in molecules usingelectrons coherently rescattered during theprocess of high harmonic generation - XISRS

• Preliminary theoretical models agree withexperiment

• Probing attosecond dynamics in materialsSF6

CClF3

• Pump laser field can align molecules

X-ray generation from molecules

Itatani et al., Nature 432, 867 (2004)

• Since strength of emission depends onorientation, have a new probe of staticmolecular tomography(Velotta, et al, PRL 8718 (18) (2001))

• Probe pulse can generate harmonicsfrom aligned molecules

Can we observe intramolecular dynamics?

• Is harmonic generation sensitive to small amplitude vibrationalmotions in a molecule? What modes can we observe?

Exciting molecular dynamics using lasers

K. Nelson, Science 247, 1317 (1990)Weinacht, et. al, Chem. Phys. Lett. 344, 333 (2001)

Bartels, et al., Phys. Rev. Lett. 88, 123 (2002)

What happens when we hit a molecule witha short light pulse?

I. Molecule can alignalong the field

II. Excite Raman-active vibrations

Experiment

• IR pump pulse excites vibrations using Impulsive Stimulated Raman excitation(ISRS) with Tpump < Tvib

• IR probe pulse excites harmonics from vibrational wavepacket with Tprobe < Tvib

PNASto be publishedAug 2006

Time delay (ps)

Har

mon

ic O

rder

Observe modulation in the high harmonic emission

• Observe oscillations in all harmonic orders vs. pump-probe delay• Period of oscillations ≈ molecular vibrations

with pumpno pump

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 00 . 0 0

0 . 0 5

0 . 1 0

0 . 1 5

0 . 2 0

0 . 2 5

Peak

-to-P

eak

Am

plitu

de (%

)

W a v e n u m b e r s ( c m-1 )

Fourier analysis of oscillations -> vibrational spectrum

• Observe three distinct peaks in the fourier transform• High-order Impulsive Stimulated Raman Scattering?

47th Harmonic 15.8THz

19.5THz

22.7THz

pump on

pump off

Wavenumber (cm-1)

Vibrational modes in SF6

E (cm-1) Type Assignment Freq (THz) T (fs) Comment 351 Raman 6(ƒ2u) f. 10.53 94.96 Very weak 525 Raman 5(ƒ2g) 15.75 63.49 Weak 615 Infrared 4(ƒ1u) 18.45 54.20 Very strong 642.3 Raman 2(eg) 19.27 51.90 Weak 774.5 Raman 1(a1g) 23.23 43.04 Very strong 948.1 Infrared 3(ƒ1u) 28.44 35.16 Very strong

• SURPRISE -- observe ALL the Raman-active modes

from “Infrared and Raman Spectra of Polyatomic Molecules”, Herzberg

775cm-1

43fsstrong

643cm-1

52fsweak

525cm-1

63fsweak

Forbidden

Probe vibrations using visible Raman scattering

• Use spherically-symmetric, Raman active molecule: SF6

• Tpump < Tvib -- Impulsive Raman excitation (ISRS)• Tprobe < Tvib -- measure Raman scattered narrow band visible

T

Short, impulsive,pump pulse at ω

Long, weak, probe pulse at 2ω

200 400 600 800 1000 12000

2

4

6

8

10

12

14

!2, 643 cm

-1

!1, 775 cm

-1

Pe

ak

-to

-pe

ak

am

pli

tud

e (

%)

Wavenumber (cm-1)

with pump pulse without pump pulse

!5, 525 cm

-1

Direct comparison with visible-probed ISRS

VisibleRamanprobe

X-ray probe

200 400 600 800 1000 1200

Inte

nsit

y (

arb

.)

Wavenumbers (cm-1)

with pump pulse without pump pulse

!1, 775 cm

-1

• Conventional ISRS ONLY seessymmetric breathing mode

– 100x stronger than other modes(Chem. Phys. Lett. 344, 333 (2001); Phys.Rev. Lett. 88, 123 (2002))

• Ultrafast Raman is x103 less sensitivethan x-ray signal because we needed tobackfill entire chamber to see thevisible signal!

• Harmonic emission MORE sensitive toMORE modes!

Observe dynamics in non-spherically symmetric molecule - CClF3

• CClF3 is a non-spherically symmetric molecule with six normal modes

• Observe larger modulation of the HHG signal due to vibrations than SF6

CClF3(Freon 13)

Time delay (ps)

Har

mon

ic O

rder

file:///Users/margaretmurnane/Desktop/Temporary%20Work%20June%2006/Data/Nick%20data/freon%20animations/CF3Cl_mode

3.gif

Observe dynamics in non-spherically symmetric molecule - CClF3

• CClF3 is a non-spherically symmetric molecule with six normal modes

• Observe larger modulation of the HHG signal due to vibrations than SF6

-0.5 0.0 0.5 1.0 1.5 2.0

0

200

400

600

800

1000

1200

1400

41st

Har

mon

ic In

tens

ity (a

rb.)

Time delay (ps)Time delay (ps)

CClF3(Freon 13)

Spontaneous Raman spectrum of CClF3

Claassen, J. Chem. Phys. 22, 50

Vibrational modes in CClF3

• C Cl F3 has six normal modes with a range of periods (27 - 95 fs)

• Would not expect to see high-frequency modes since period ≈ pulsewidth

Wavenumber (cm-1

) Period (fs)

350 e fundamental 95

468 a1 fundamental Cl37

71


70


781 a1 fundamental 43




• Observe two strongest Raman-active modes

• Likely due to signal-to-noise and pump pulse limitations

Wavenumber (cm-1

) Period (fs)



71


70





200 400 600 800 1000 1200 14000

2

4

6

8

10

Peak

-to-P

eak

ampi

ltude

(%)

Wavenumber (cm-1)

781 cm-1

476 cm-1

468 cm-1


• Observe two strongest Raman-active modes

• Likely due to signal-to-noise and pump pulse limitations

200 400 600 800 1000 1200 14000

2

4

6

8

10

Peak

-to-P

eak

ampi

ltude

(%)

Wavenumber (cm-1)

781 cm-1

476 cm-1

468 cm-1

Wavenumber (cm-1

) Period (fs)



71


70





Why does HHG probe of vibrations differ from ISRS?

• Not likely due to excitation - visible and x-ray experiment had same pump pulse

• Wavelength of recolliding electron comparable to molecular dimension

• Quantum interferences thus make HHG very sensitive to the shape of the molecule

!

"e = hp

= h2meE

= .15nm@ H45 (70eV )

Quantum picture - electron being ripped from atom

Physics Today, Kapteyn et al. March 2005

time

laser

Electron wavefunction

2D Plane wave electron recollision with SF4

Simulation by Ivan Christov

time

HH

G si

gnal

Theoretical understanding to date

• Fully quantum calculation of simplelinear triatomic molecule (IvanChristov)

• Predict that both symmetric andantisymmetric modes observable -harmonic generation should besensitive to ALL modes, both Ramanand IR-active

• Predict 2mÅ sensitivity for currentexperiment (0.1% modulation in bondlength)

symmetricanti-

symmetric

Time (IR periods)

v=0 v=0

v=1

Ramanpulse time T

Probing vibrational Raman-excited quantum beatsZ. B. WaltersS. TonzaniC. H. Greene

v=0 v=0

v=1

Ramanpulse

v=1

v=0e+ion

e+ion

Ionizingpulse

time T


v=0 v=0

v=1

Ramanpulse

v=1

v=0e+ion

e+ion

Ionizingpulse

time T Recomb.uponrecollision

v=0

v=1

Electron propagation ≈ fs


v1

v2

v5

New spectroscopic high-order x-ray Raman probe?

XISRS?

ω 39ω

ωvib

Vibrations most visible in higher harmonics

• All vibrations more visible for higher harmonic orders

• Higher harmonic orders correspond to shorter wavelengths of therecolliding electrons and shorter duration x-ray pulses

ν1 symmetric

ν5 asymmetricν2 asymmetric

Harmonic order

Wav

enum

ber

(cm

-1)

Observation of vibrational and reorientational relaxation

• Asymmetric mode decays over time interval investigated, possibly due toreorientational dynamics

• Amplitude of symmetric mode remains constant

200 400 600 800 1000 12000.00

0.02

0.04

0.06

0.08

0.10

0.12

Pe

ak

-to

-pe

ak

am

pli

tud

e (

%)

Wavenumbers (cm-1)

Limits of DFT Center Length

0.45 ps 0.3 ps 0.75 ps 0.3 ps 1.05 ps 0.3 ps

!5, 525 cm

-1

!1, 775 cm

-1

early

later

Vibrational dynamics in C Cl F3

• Observe rapid decay of 470cm-1 and 560cm-1 modes and persistence of 781cm-1 mode

200 400 600 800 1000 12000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

FT

am

plit

ude

41st h

arm

onic

Wavenumber (cm-1)

First Middle Last

Earlyin

time

Laterin

time

-0.5 0.0 0.5 1.0 1.5 2.0

0

200

400

600

800

1000

1200

1400

41st

Har

mon

ic In

tens

ity (a

rb.)

Time delay (ps)Time delay (ps)

e-EUV pulse

Time-of-Flightdetector

sample

ωEUV

Photoelectron

Eel = ωEUV - I.P.

EUV Photoemission in the Presence of an Intense Laser

• EUV pulse photoionizes the gas atoms• Laser field modifies photoelectron energies (Glover et al. PRL 76, 2468 (1996)

EUV Photoemission in the Presence of an Intense Laser

Infrared pump

EUV probee-

Time-of-Flightdetector

sample

• EUV pulse photoionizes the gas atoms• Laser field modifies photoelectron energies (Glover et al. PRL 76, 2468 (1996)

ωEUV

ωIR

EUVprobe

Dressed sideband PE peaks from atoms

Continuous PE spectrum from solids

• How to resolve sideband structure in laser-assisted photoemission fromsolids in order to observe attosecond dynamics in solids?

• The photoelectron spectra from clean Pt(111) exhibit a narrow d-bandpeak at the Fermi edge - that is ideal for the observation of sidebands

Can we see laser assisted photoemission from surfaces?

solids

Photoelectron spectra

atoms

p-polarized IR pump

probe-only

Photoelectron spectra from Pt in presence of IR pump

0 fs

• Photoelectron spectradramatically modulated in thepresence of an IR laser

• Not hot electrons!!

Modulation of photoemission consistent with dressing

1200

1000

800

600

400

200

0

counts

4035302520

electron energy (eV)

0.6

0.4

0.2

0.0

!""

2#

$

f(E

-E0

)

-4 0 4E-E0 (eV) pump-probe

probe only fit

Sideband fit to d-band photoelectron peak explains data:

0 fs

Sideband response function with fit parameters:1.0

0.8

0.6

0.4

0.2

0.0

!""

2# $

f(

E-E

0)

-4 -2 0 2 4E-E0 (eV)

without IRwith IR

Cross-correlation of EUV and IR beams

0.25

0.20

0.15

0.10

0.05

0.00

sid

eband h

eig

ht (a

.u.)

-100 -50 0 50 100

Time Delay (fs)

FWHM= 37 fs

• Magnitude of sidebands yields FWHM of EUV pulse at 37 ± 3 fs (limited by IRpulse duration)

• Sub-femtosecond time resolution is feasible - opening up measurements ofcomplex, attosecond, electron dynamics in solids and adsorbates

• Method useful to characterize high energy harmonics, where atomic mediumwould have too low a cross section

This REALLY is laser-assisted photoemission from surfaces

• No sidebands observed for perpendicular polarization

• Sideband intensity proportional to laser intensity

• Generation and observation of hot electrons occurs at higher laser intensity

• Excellent agreement of fit to experimental data

0.30

0.25

0.20

0.15

0.10

1st ord

er

sid

eband a

mplit

ude

181614121086

pump energy ( µJ/pulse)

Conclusion and Future Plans

• Utility of harmonic generation as a new high-order Raman probe of intra-molecular dynamics is immediately apparent

– High-order x-ray Raman scattering sensitive to ALL vibrational modes– SIMPLE fourier transform of data yields vibrational modes– > 1000 times more sensitive than conventional impulsive Raman spectroscopy– Sensitive to < 0.1% changes in bond– Coherent, time resolved probe of intramolecular relaxation on ground state potential

surface

• Future work– Resonant excitation and dissociation using ultrashort VUV pulses– Non-adiabatic transitions (i.e. internal conversion and intersystem crossing)– Attosecond dynamics in molecules

• Thanks to DOE and NSF!

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